The use of borate salts in imaging materials is known. For example: U.S. Pat. Nos. 4,343,891, 4,447,521 and 4,450,237 disclose the use of borate salts with cationic dyes as photoinitiators for polymerization in colour imaging media and magnetic recording media; U.S. Pat. No. 4,859,572 discloses the use of borate salts as electron donors for dye sensitised imaging systems; Japanese Patent No. 1-012143 discloses the use of borate salts with triazene compounds as high-speed photopolymerization catalysts; Japanese Patent No. 1-013141 discloses the use of borate salts with onium salts; Japanese Patent No. 2-034603 discloses the use of borate salts as high-speed polymerization catalysts, and Japanese Patent No. 2-058501 discloses the use of borate salts as antifoggants for photothermographic materials.
Japanese Patent Publication No. 2-240648 discloses photothermographic elements comprising a support having coated thereon a layer of a photosensitive medium comprising silver halide, a developer, a polymerisable compound and an organoborate salt. The photothermographic elements are exposed in an image-wise fashion to form a latent image and then heated to fully develop the image. The heat causes the polymerisable compound to polymerize in the exposed regions of the element. The photothermographic elements find particular utility in thermal-dye-transfer imaging, in which the thermally-mediated transfer of dye or other colourant to a receptor sheet is prevented in the polymerized (exposed) regions of the element. In the majority of the -examples, the photosensitive medium is prepared by dissolving the organoborate salt, together with the reducing agent in an organic solvent, e.g., dichloromethane and adding the organoborate/developer solution to a solution of the polymerisable compound and a colourant and dispersing the resulting mixture in an aqueous gelatino preparation of silver halide and silver benzotriazole. The photosensitive composition is then emulsified in an aqueous pectin solution and treated with alkali to prepare photosensitive microcapsules therefrom. The conditions of formulation are such that the organoborate salt, namely tetrabutyl ammonium tetrabutylborate, which is substantially insoluble in aqueous solution, is confined to the hydrophobic organic phase thereby preventing it from contacting the silver halide grains.
In one example (Example 3), the photosensitive element is prepared by coating the support with a first layer comprising silver behenate and a polymerizable compound which is then dried and overlaid with a second layer comprising ammonium halide, the organoborate compound and a hindered phenol developer. The exposed element is heated, typically for 20 seconds at 110.degree. C., and then `wet-processed` by immersion in a methanol bath to remove the photosensitive medium in the non-irradiated regions.